U.S. patent number 5,566,354 [Application Number 08/312,271] was granted by the patent office on 1996-10-15 for system and method for channel assignment in a satellite telephone system.
Invention is credited to Jerry R. Sehloemer.
United States Patent |
5,566,354 |
Sehloemer |
October 15, 1996 |
System and method for channel assignment in a satellite telephone
system
Abstract
A system and method for making channel assignments in a low
earth orbit satellite telephone system wherein Doppler effect is
utilized as a parameter in channel assignment. In a first
embodiment the satellites orbit in a fixed grid pattern, and in a
second embodiment the satellites move in orbits that appear random
relative to each other.
Inventors: |
Sehloemer; Jerry R. (Round
Lake, IL) |
Family
ID: |
23210675 |
Appl.
No.: |
08/312,271 |
Filed: |
September 26, 1994 |
Current U.S.
Class: |
455/427;
455/13.1; 455/450; 455/513 |
Current CPC
Class: |
H04B
7/18539 (20130101); H04B 7/18597 (20130101); H04B
7/2123 (20130101) |
Current International
Class: |
H04B
7/212 (20060101); H04B 7/185 (20060101); H04B
007/185 () |
Field of
Search: |
;455/12.1,13.1,13.2,33.1,54.1,56.1,62,63,67.1,98 ;342/352,418
;379/59 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Eisenzopf; Reinhard J.
Assistant Examiner: Banks-Harold; Marsha D.
Attorney, Agent or Firm: Aubel; Leo J.
Claims
I claim:
1. A method for making channel assignments in a satellite
communications system comprising a plurality of low earth orbiting
satellites, a plurality of land bases each including transmitter
and receiver means operable in plurality of communication channels,
and a plurality of mobile units (remotes) each having transmitter
and receiver means which are selectively assigned to operate in
said plurality of channels; said method being effective to make
interference free channel assignments for remotes incoming into
said system to communicate with selected proposed satellites
without the use of central controllers, said method including the
steps of:
a) making tentative channel assignments between remotes and
proposed satellites based on actual signal strength;
b) testing by a proposed satellite as to whether its transmission
on a tentative channel would interfere with a transmission on a
channel which a previously assigned remote is utilizing which
channel may be different from said tentative channel due to Doppler
shift, wherein the proposed satellite includes means for sensing
Doppler effect;
c) determining by an incoming remote being assigned to said
tentative channel whether it would be interfered with by a
previously assigned satellite utilizing said tentative channel;
d) determining by said incoming remote whether it would interfere
with a previously assigned satellite on the channel that the
previously assigned satellite is utilizing which channel may be
different from said tentative channel due to Doppler shift,
e) determining by said proposed satellite whether it would be
interfered with by a previously assigned remote, and
dependent on the results of said tests, initiating
communications.
2. A method as in claim 1 including the step of making channel
assignments dependent on the Doppler shift, and making said
assignments on a priority hierarchy starting from the most positive
Doppler shift and moving to the most negative Doppler shift.
3. A satellite communications system comprising a plurality of low
earth orbiting satellites which satellites are orbiting in paths
which are random relative to each other, a plurality of land bases
each including transmitter and receiver means operable in a
plurality of communication channels, and a plurality of mobile
units each having transmitter and receiver means which are
selectively assigned to operate in said plurality of channels;
a) means for making tentative channel assignments between mobile
units and proposed satellites based on signal strength;
b) each satellite including means for sensing Doppler effect and
means for testing whether its transmission on a tentative channel
would interfere with a transmission on another one of said channels
which a previously assigned mobile unit is utilizing, which said
another one of said channels may be different from said tentative
channel due to said Doppler effect.
4. A system as in claim 3 wherein,
a) said bases include means for sensing Doppler shift, and each
base including means for testing whether its transmission on said
tentative channel would interfere with a transmission on another
one of said channels which a previously assigned satellite is
utilizing which said other one of said channels may be different
from said tentative channel due to Doppler shift.
5. A system as in claim 3 wherein
a) said mobile units include means for sensing Doppler shift, and
each mobile unit includes means for testing whether its
transmission on said tentative channel would interfere with a
transmission on another channel which a previously assigned mobile
unit is utilizing which other channel may be different from said
tentative channel due to Doppler shift, and said mobile units can
select the base with which said mobile units with which they will
communicate.
Description
BACKGROUND OF THE INVENTION
The explosive growth of cellular radio during the late 1980s and
early 1990s highlighted the importance of mobile telephone
communications. Almost all large world wide cities in the developed
world now enjoy some form of cellular communications. However,
there are still some areas that are not served by cellular. In some
developing nations there is not enough of a demand to install wide
area cellular systems. In developed nations, there are still remote
areas that have not yet been covered by the cellular system.
One approach that has enjoyed considerable press is the idea of
creating a satellite system that would provide telephone service
the world over. The argument for such an approach is that it would
provide coverage to developing countries, and also provide coverage
in remote areas in developed countries. (In addition many land line
phone companies have less than stellar reputations in some
countries, and it is assumed that many citizens would also use a
newer and more reliable service.)
In addition, the cost of cellular radio is quite high, and there is
a good possibility that satellite systems would yield a lower cost
to the final user. Some of this cost savings will be from a
simplified business structure. Today, a cellular user might begin
his call with a local cellular operator that switches his call to
the local bell telephone company that further processes and
switches his call. Perhaps a single satellite system could route
calls to the final user with fewer different business involvements
that would lead to a lower cost.
The physics of satellite orbits, and the physics of radio
propagation are very important in the design of a satellite radio
telephone system. Radio propagation is such that the available
higher frequencies for such a system will not bend around the
curvature of the earth, and that such systems will be generally
limited to line of sight communications.
The first approach that would normally be considered is to place a
geostationary satellite in orbit high above the earth. The first
satellite radio links between the U.S.A. and Europe used this
approach. There are several weaknesses to this approach:
1) The very high altitude of geostationary satellites causes
limited frequency reuse in the system. Since spectrum is a valuable
resource, such high altitude satellites are very spectrally
inefficient in that the channels could only be used once in a very
large area.
2) The very high altitude of geostationary satellites implies that
there is a considerable distance between the phone user and his
satellite. This large distance requires large transmitter power
and/or a large antenna system at both ends of the communication
link. These two limitations are especially difficult for the
roaming mobile telephone user.
A second more practical approach is to place a pattern of orbiting
satellites into lower earth orbits. These multiple satellites would
permit some frequency reuse, and these lower altitude satellites
would be closer to the earth surface, and thus smaller antennas and
less power would be required. This approach of low earth orbit
satellites has received considerable press, and they are frequently
referred to as LEOS.
Several consortiums of large companies have proposed various
approaches built around LEOS. The strategy would be to use the
cellular radio concept of not using the same channel on adjacent
satellites, and yet to create a grid of orbits such that coverage
of the earth would be guaranteed. One proposed grid, however, would
have orbits that rotate equally north and south of the equator and
thus avoid the poles and simultaneously enhance coverage in the
regions where the people reside. The Motorola Iridium concept,
however, has orbits that rotate in a longitudinal manner around the
earth.
U.S. Pat. No. 5,274,840 issued to Motorola shows a grid of 48
different satellites in 6 different planes. There are 8 satellites
in each plane. The planes all intersect near the axis of rotation
of the earth. All 48 satellites orbit over both poles. U.S. Pat.
No. 5,161,248 issued to Motorola shows a grid of 77 satellites in
similar polar orbits. However, in this system there are 7 planes
with 11 satellites in each plane.
U.S. Pat. No. 5,119,504 issued to Motorola explains an anticipated
handoff system. Since the satellites know where they are located,
and the satellites also know the location of the earth user, they
can calculate based on knowledge of their own orbits when handoffs
are required.
Although these approaches are technically sound, the cost to create
one of these systems is estimated to be near four billion dollars.
In fact one recent announcement was budgeted at about 8 billion
dollars. Some of the various factors that are contributing to the
cost of such as system are as follows:
P1 Approximately 80 satellites are utilized in the present system.
Each of these satellites weighs approximately 1,500 pounds. This
results in a very expensive satellite, and also in a very expensive
rocket launch cost. A total of 105,000 pounds of very high
technology equipment has to be put into orbit.
In addition to the expensive and heavy satellites there is a very
expensive satellite orbit control system to keep the satellites in
their proper orbits. Each satellite needs various rockets, rocket
fuel, and orbit control computer/radio technology to keep the
satellite in a proper orbit. In case one of the satellites would
accidentally end up in an improper orbit, there is a very
comprehensive ground computer control system to insure that all the
satellites stay in correct orbits, and to disable a satellite when
it is not in the proper grid orbit. In addition, the complete
failure of any one satellite would require the moving of a back up
unit to take its place in the grid.
SUMMARY OF THE INVENTION
This invention provides a new approach to channel assignment in a
LEOS system by using signal strengths adjusted by Doppler effects
to calculate assignments based on actual signal strengths without
relying on satellite position. This approach provides both the
satellites and the earth user with a better signal.
DESCRIPTION OF DRAWINGS
The foregoing features and advantages of the present invention will
be apparent from the following more particular description of the
invention. The accompanying drawings, listed hereinbelow, are
useful in explaining the invention.
FIG. 1 is a diagram showing remotes, base sites, and satellites:
and
FIG. 2 is a diagram showing the signal paths that comprise a
conversation between a remote and a base site using a
satellite.
FIG. 3 is a sketch that shows a typical pattern of satellites
having random orbits relative to each other with respect to a
mobile user:
FIG. 4 is a sketch that indicates Doppler shift differences
relative to an earth antenna based on relative position of the
satellites:
FIGS. 5(a), 5(b), and 5(c) show a flow diagram that explains a
procedure to determine if a satellite can choose a channel for
initial broadcast:
FIGS. 6(a) and 6(b) show a flow diagram that explains a procedure
to determine if the remote will receive a good signal on proposed
channel:
FIGS. 7(a) and 7(b) show a flow diagram that explains a procedure
to determine if the new remote will interfere with a previously
assigned satellite:
FIGS. 8(a) and 8(b) show a flow diagram that explains a procedure
to determine if the new satellite will hear a good signal from the
new remote:
FIGS. 9(a) and 9(b) show a flow diagram that explains the handoff
and pre-selected base site procedure:
FIG. 10 is a sketch that shows the distance to horizon concept:
FIG. 11 is a sketch that shows a typical grid pattern of satellites
over a mobile user:
DETAILED DESCRIPTION OF THE INVENTION
Referring first to FIG. 1, the present inventive satellite system
21 comprises base sites 22 that connect into the land line
telephone network as is well known, a number of earth orbiting
satellites 23, and multiple users 24. Users may be in vehicles, may
be in buildings, or may be carrying portable handheld units: these
user units are herein referred to as remotes or mobiles.
Referring to FIG. 2, the present inventive satellite system is a
full duplex system in that each mobile unit 24 uses a pair of
channels 25a and 25b to communicate with the satellite. The
satellite uses a pair of channels 26a and 26b to communicate with
the base station 22.
There are several different choices for frequencies for a satellite
system. Low frequencies tend to bend around obstacles and provide
very good service. On the other hand, higher frequencies are very
subject to shadow effects and do not yield good signals to a
receiver. At very high frequencies such as 10,000 megahertz to
20,000 megahertz, spectrum capacity is not an important issue
because of the large bandwidths available. However, at these high
frequencies the reception will be severely limited by trees,
clouds, and building shadows.
At frequencies around 1,800 megahertz radio reception will be
satisfactory. Therefore, the present system utilizes frequencies in
the 1,800 megahertz band.
Potential users of the satellite system will be in cars with steel
roofs and in homes with wood and conventional shingle roofs. Other
users will be in commercial buildings with reinforced steel roofs.
Other users will be shielded from line of sight to their satellites
by foliage. It is difficult to express numerically the amount that
such obstacles will reduce received power. However, the power
transmitted must be sufficient to overcome these obstacles. The
lower the frequency, the better will be the resulting service.
Consequently, a band of frequencies near 1,800 megahertz is
effective.
The mobile units in the present system transmit at 400 milliwatts,
and the satellites transmit at 400 milliwatts. The earth stations,
which are stationary, and do not have battery constraints transmit
at 10 watts.
The satellite system utilizes a band of 42 megahertz. 10 megahertz
are used for talking from the satellites to mobile units, and 10
megahertz are used for talking to the satellites from mobile units.
Another 10 megahertz is used for the satellites to talk to the land
line network, and another 10 megahertz is used for the land line
network to talk up to the satellites. A channel spacing of 15
kilocycles is used. As shown in FIG. 2, two different links are
required to make a phone call. The satellite user 24 uses a duplex
pair of channels 25a and 25b to talk up to the satellites, and the
satellite 23 uses another pair of duplex channels 26a and 26b to
return the conversation to the base station 22 on earth. Therefore,
a complete communication between the mobile and the satellite
requires four different links. These links require four bands each
of 10 megahertz for a total band of 40 megahertz.
The following are the operating system parameters:
______________________________________ Number of Satellites in
Orbit 80 Units Typical Height of a Satellite 500 Miles Number of
Base Sites 1 Minimum Operating Frequency of System 1,800 Megahertz
Spectrum set aside for total system: 42 Megahertz Spectrum for up
link from the remote 10 Megahertz Spect. for down link from
satellite to remote 10 Megahertz Spect. for up link from land line
network 10 Megahertz Spect. for down link from satellite to land 10
Megahertz Spectrum for paging satellite to satellite 1 Megahertz
Spectrum for paging satellite to ground 1 Megahertz Channel spacing
15 Kilocycles Guard Bands at end of each block 50 Kilocycles Number
duplex channel pairs in the system 650 for satellite to remote:
(650 channels talk up, 650 channels down) Number of duplex channel
pairs in the system 650 for satellite to land line (650 channels
talk up, 650 channels down) Duplex channel pairs required to per
call 2 Number of Satellites in initial launch 80 Number duplex
pairs per satellite (mobiles) 40 Number duplex pairs per satellite
(landline) 40 Times a channel typically is reused 5 Paging channels
(single) 40 ______________________________________
Since it takes two duplex sets to support a telephone call, each
satellite supports up to 40 simultaneous phone calls. Broad band
amplifiers are currently in development that will increase the 40
conversation capacity of each satellite. As explained in U.S. Pat.
No. 4,965,850 issued to the inventor hereof, frequency reuse can be
increased by a factor of three by selected various combinations of
sites and mobiles.
The call handling capacity of the satellite system is limited. The
fact that satellites require two sets of channels detracts from
their potential capacity. Also in comparison, in land based
cellular, most systems are designed that a channel may be reused up
to five or ten times in a single metropolitan area. For example, in
Chicago metropolitan area one might be able to reuse a channel up
to five to ten different times. The limited antenna heights, and
the tightly designed cellular patterns are created such that a
channel intentionally can be reused many times. In addition, the
same channels being used Chicago are used in Milwaukee and even in
Springfield and St. Louis. Consequently, in a typical land based
cellular system channels might be reused up to 20 or 40 or more
times in a radius of 300 miles.
However, in a satellite system frequency reuse is limited, and
consequently, a channel can be used only once in a very large area.
Therefore, from the outset it must be understood that satellite
systems are generally not an effective use of the radio
spectrum.
Call Set Up and Handoff Procedure
In land based cellular systems the cell sites are laid out such
that adjacent sites can not use the same channels. In this simple
manner, adjacent channel interference is eliminated. However, by
permitting distantly located sites to use the same channel,
frequency reuse is permitted and spectrum capacity is enhanced.
In satellite systems where the satellites orbit in a grid or
pattern, adjacent satellites are not permitted to use the same
channel. Call set up operates in a manner similar to the land based
system. Users search for a strong signal, and end up communicating
with the satellite closest to them. The fact that certain
satellites communicate only on certain channels guarantees that
simultaneous channel usage on adjacent satellites will not
occur.
Referring to FIG. 11, this diagram shows part of a grid of
satellites in orbit, and shows that the few satellites above a
given earth location will be in a pattern.
It has been found that assignments based on signal strengths in
land based systems with a grid of base sites approximately triples
the number of potential conversations. This technology has been
disclosed and claimed in U.S. Pat. No. 4,965,850 cited above.
We have found that certain related signal strength technology is
applicable to the present satellite system. Adding signal strength
assignment improves the system and will not significantly change
the cost to design, build, and launch the satellite system. Signal
strength assignment in satellite systems will approximately triple
the call handling capacity of the inventive system.
The following procedure is used for call set up. Refer to U.S. Pat.
No. 4,965,850 which is hereby incorporated by reference. U.S. Pat.
No. 4,965,850 explains various concepts that are useful in the
understanding of this invention such as signal to noise, FM
capture, and frequency reuse. The modulation method of the
satellite system is narrow band frequency modulation: this
modulation method supports both voice and data communication. With
voice conversations TDMA (Time Division Multiple Access) is used.
Because of the natural pauses in voice communications and the low
information content of voice, it has been found that voice can be
chopped up into smaller time segments, and that several of these
conversations can be placed on a single radio channel. As stated
above, in the present system channels are only used in pairs. For
each satellite transmitter channel for transmitting to a remote,
there is a corresponding channel for transmitting from the remote
back to the satellite. The satellite to earth station channels are
also used in pairs. Instead of referring to channels by their
actual frequency, channel pairs will be referred to as 1, 2, . . .
650 for the satellite to remote group, and 1, 2, . . . 650 for the
satellite to land line network or earth station. For each channel
the group designation will be given. Each group has 650 channels as
shown in the table above.
Only the mobile (remote) users can initiate calls. Land line users
desiring to call a mobile (remote) user will page the user, and let
the mobile (remote) user set up the call.
The paging process is limited to situations where the calling party
knows the approximate location of the desired mobile user. The
initiating caller will phone the land base nearest the assumed
location of the mobile user, and that land base will use the
satellites in the vicinity as transponders to copy the land base
initiated page. The land base sends the page signal to all the
satellites in the vicinity, and they merely copy, transpond, the
page.
The transmitters of the remotes, bases, and satellites can only
transmit on the given channels, and can not be tuned to transmit in
the continuum between the channels. These frequencies are
designated as channel 1, Channel 2, etc. As will be explained
herein after, the receivers of the bases, remotes, and satellites
will track the signals, and will consequently receive signals on
the channels, and also in the continuum between the designated
channels as will be explained hereinbelow.
It is known that sound from a source which is moving towards the
listener will arrive at a higher frequency than was transmited;
and, that sound from a source that is moving away from the listener
will arrived at a lower frequency than was transmitted. This
phenomenon is known as a Doppler Shift.
In radio systems, electronic waves travel at the speed of light.
Consequently, the Doppler problem is only of concern in radio
systems where the velocities involved are very high. The frequency
shift caused by Doppler in a radio system can be calculated by
multiplying the transmitted frequency by the ratio of the relative
velocity divided by the speed of light. It is interesting to note
that the Doppler problem becomes more severe at higher frequencies.
Because the Doppler shift in any low earth orbit satellite system
will cause special design considerations in the receivers, this
could be called the Doppler problem. However, in the invention
disclosed herein the Doppler problem is converted to a "Doppler
Opportunity".
Because of the high speed of the satellites there is a Doppler
shift in the received frequencies. Because the relative speed of
the satellites with respect to a given point on earth is also
changing, the magnitude of this Doppler shift is also changing.
Because of the continuously changing received frequencies in the
system that are always gradually changing due to different relative
velocities between the given point on earth and satellite, the
receivers in the system will track the actual frequency being
received. The center frequency of the input band of the receiver
will stay tuned to the center frequency of the received signal. For
example, a conversation might begin with a receiver listening on
channel 8, and at the close of the conversation, the receiver could
be listening on channel 9. In the interim, the receiver could be
listening half way between the center of channel 8 and the center
of channel 9. For explanation purposes, this half way position will
be referred to as channel 8.5. During the tracking process the
center of the receiver band will move continuously from 8.1 to 8.2
. . . to 8.9.
Because of a Doppler shift in the system that is based on the
relative speed of the satellite to the earth located remote or the
relative speed of the satellite to the earth located land line
network base, the actual frequencies being transmitted, and the
actual frequencies being received can vary as much as three
channels. In the system described at a frequency band of 1800
megahertz, this Doppler shift corresponds to 45 KC. This 45 KC
corresponds to three channels. If the satellite is moving directly
toward an earth receiver, the frequency received will be increased
as much as three 15 KC channels. If the satellite is moving
directly away from an earth receiver, the frequency received will
be decreased as much as three channels. If the satellite is moving
exactly perpendicular to an earth station, the frequency received
will not be different from the frequency transmitted. Depending
upon the actual relative velocity of the satellite to the earth
station, the frequency shift can be anywhere between plus or minus
three channels.
Refer to FIG. 4. In this diagram the satellite 23 on the left is in
orbit traveling at approximately 18,000 miles per hour. Hour, since
the satellite 23 is moving somewhat tangential to the earth station
(which may be either the remote 24 or the base site 22), the
relative velocity between the earth station and the satellite is
only 12,000 miles per hour. The relative velocity of the satellites
to the earth locations is calculated by breaking the satellite
velocity into two different vectors. One of these vectors points
towards the earth location, and the other vector is perpendicular
to the line to the earth station. The relative velcoity is
calculated by the following formula.
Where the Vel stands for the relative velocity of the sattellite to
the earth location.
This 12,000 miles per hour will create a Doppler shift of about two
channels. Consequently, when the satellite transmits on channel 47,
the earth site will hear a signal on channel 49. However, the
satellite directly overhead is also traveling at 18,000 miles per
hour. However, the relative velocity of this satellite to the earth
position is zero. Consequently, a signal that is transmitted on
channel 47 arrives on channel 47. Consequently, in this diagram,
two satellites could both be transmitting on channel 47, and yet an
earth site could be receiving a single clear signal on channel
47.
All existing conversations add a tone or digital data stream to
their conversation indicating the signal strength that is being
received. Also all conversations add a tone or digital data stream
to all transmissions indicating the actual channel number being
used for transmission, and the actual channel number being used for
receiving.
Consequently, each ongoing communication in the system will have
the following information continuously added to the
conversation:
1. Signal strength being received
2. Actual channel being used for transmission
3. Actual channel plus decimal fraction being received.
This information is utilized in the signal processing and
computational method that makes channel assignments.
For example, a satellite near the horizon coming toward both a
ground user, and the ground station might have as follows:
In the communication to the remote:
______________________________________ 1. Signal strength 30 DB 2.
Transmit on channel 45 3. Receive on channel 46.5
______________________________________
In the communication to the land line base station:
______________________________________ 1. Signal strength 45 DB
(Better Base antennae) 2. Transmit on channel 31 3. Receive on
channel 32.4 ______________________________________
Because large numbers of remotes and satellites are involved in the
following discussion, the following terminology is used:
______________________________________ Old Satellite For a
satellite already in communication Old Remote For a remote already
in communication New Satellite A satellite that could communicate
New Remote A remote that wants to communicate Old Land Base A land
line base in communication New Land Base A land base that could
communicate ______________________________________
There are also two bands of frequencies involved.
______________________________________ Remote Band Remotes to
satellite/satellite to remote Base Band Base to
satellites/satellite to base
______________________________________
The call set up procedure as will be explained herein is divided
into two parts. Part I will connect the new remote to the
satellite, and Part II will connect the satellite to the new ground
base station. In each part there are four steps.
Part I
Establish new satellite to new remote channel assignments
The following technology is related to that disclosed in U.S. Pat.
No. 4,965,850 issued to the inventor hereof, and that patent is
incorporated herein by reference.
Step IA.
Will the new satellite transmission in the remote/satellite band of
frequencies interfere with an old remote unit? Refer to FIG. 5 that
shows a flowchart of the system logic for the following step
1A.
Before a new satellite even begins the following process of
transmitting that it has a tentative channel, the new satellite
monitors the band upon which the various earth stations are
transmitting. If the new satellite does not hear any earth
stations, the new satellite assumes that it is over an area without
access to a ground station. The new satellite would then not begin
the following transmissions and would remain silent.
The new satellites are sequentially examining all communications
channels in the remote band. For example, let us assume the new
satellite is examining channel 43 as a potential channel for
transmission to carry on a conversation with a new remote. The new
satellite receiver scans the spectrum in a continuous way from
channel 40 to channel 46. This scanning process is well known and
is similar to the scanning process that occurs when a radio
listener searches an AM radio band for stations with good music.
The new satellite discovers an ongoing communication. Let us assume
that the new satellite hears this communication on channel 40.4.
i.e., the new satellite hears a communication between channel 40
and channel 41.
The new satellite then notes the following information:
A. The new satellite hears a signal on channel 40.4
B. This transmission originated on channel 38.
C. The old remote is receiving on channel 36.
D. The old remote is receiving a signal at 30 DB
E. The new satellite is receiving a signal at 30 DB
The new satellite observes that since this transmission originated
on channel 38, and the old remote is receiving on channel 36 that
the other pair are having a frequency decrease and thus are moving
apart.
The new satellite observes that since it heard this transmission on
channel 40.4 that originated on channel 38 that it is moving toward
the old remote.
In FM communications it has been experimentally determined that an
unwanted signal will not cause problems to a desired signal of the
signal is about 10 to 15 db weaker than the desired signal. A
safety factor is added, and test is made for a 25 db margin. Since
the Doppler will change the received frequencies during the
duration of a call, it has been determined that a signal within 15
kc of the desired signal could potentially cause problems in a
short duration. Consequently, if the undesired signal is within 15
kc of the desired signal, and if it is within 25 db of the desired
signal, it would either cause or potentially cause interference and
would not be suggested as an acceptable assignment.
The new satellite can calculate that since it hears a transmission
on channel 40.4 and the transmission originated on channel 38 that
the frequency being received from this old remote is 2.4 channels
higher than the old remote transmitted. Since the new satellite and
the old remote have velocity reproprocity, the new satellite
calculates that if it transmitted on channel 43 that this old
remote in question would hear a transmission on channel 46.4. This
46.4 channel is sufficiently distant from the channel upon which
the old remote is using to receive (channel 36) that this old
remote would not be bothered if the new satellite were to choose
this (channel 43) channel for a transmission.
The new satellite continues to scan the channels from channel 40 to
channel 46 and performs similar calculations. If the new satellite
deduces that a channel is unsatisfactory, then the new satellite
steps to the next channel as a tentative channel. The new satellite
would then attempt to use channel 44 as opposed to channel 43. As
the new satellite steps through the channels, some of these
calculations are redundant, and are eliminated. However, all the
steps are outlined here to convey the method of the invention.
When a new satellite discovers a channel upon which it will not
interfere with an old remote, the new satellite then transmits on
that channel a code in a random time slot indicating that this
channel is available. The new satellite also includes in this
transmission a number indicating the satellite's serial number. The
satellite also includes a code indicating the channel upon which it
is signaling. The satellite also includes codes indicating the
serial numbers of the various land base sites that can potentially
provide good communications links.
STEP 1B
Will the new remote receive a good signal on the proposed channel?
Refer to FIG. 6 for a detailed flow chart of step IB.
In the meantime, a new remote is desirous of setting up
communication with the satellite network. The new remote monitors
the various channels and searches for codes that indicate a channel
is available. Actually, the remote is scanning the available band,
as received signals would not land on exact channels.
Frequently, the new remote will not hear various satellite
transmissions because they are masked by ongoing conversations. The
new remote will only respond to those signal transmissions that it
hears clearly. Since the new remote finally listens on the channel
that it hears the transmission, the various Doppler shifts are
automatically taken into consideration. As an added precaution, the
remote monitors one half channel above and one half channel below
the intended channel. If the remote notes a strong signal near the
desired tentative channel, the remote ignores this tentative
channel, as the shifting caused by the Dopplers could bring this
channel into an interference causing situation.
Normally, a new remote unit will have a choice of various
satellites, and the remote must choose between a variety of
potential satellites. However, as the satellites orbit the earth,
some satellites will be approaching the remote unit, and others
will be leaving the remote unit. Others will be distant from the
mobile unit, and be on a tangential course such that they have only
a small relative velocity with respect to the remote unit. The
various relative velocities of various satellites will create a
variety of frequencies because of various Doppler effects. Refer to
FIG. 11. Satellite 55 is moving away from the remote. Because the
distance between the remote and satellite 55 is increasing, the
remote will hear a frequency less than was transmitted. Satellite
54 is moving towards the remote. Since the distance between
satellite 54 and the remote is decreasing, the remote will hear a
higher frequency than was transmitted. The new remote unit notes
the transmitted frequency of the signals from the various
satellites, and the new remote attempts to choose a satellite with
a high frequency increase (at least one channel) as a potential
channel. The remote will only choose a channel such that the
received signal is at least 20 DB over threshold. By avoiding
channels where the remote hears a weak signal, the remote only
chooses channels where the satellite is in a position to provide
good coverage. If a building or mountain is in the line of sight, a
good signal will not be received, and the remote will attempt
another channel.
For example, in FIG. 11, satellite 32 and satellite 11 will both
have increasing Doppler. However, they might be near the horizon
and might be providing a weak signal. In this particular instance,
the remote would choose satellite 55 even though it has a negative
Doppler shift.
Once a good signal is received with an increasing frequency, it is
unlikely that the position changes of the satellite would decrease
signal quality. This satellite and this channel will become the
tentative channel for communication by this remote. By choosing a
high frequency increase, the remote unit ensures that he has picked
a satellite that is moving toward him at a great velocity. That
choice would imply that the satellite is not overhead and coming
directly towards the mobile unit. Such choice selects a satellite
that will be able to serve the mobile for a long duration.
Satellites that have already passed overhead, and are leaving the
vicinity of the mobile will have a decreased frequency and such
satellites will not initially be chosen. Satellites that are on the
horizon with only a tangential velocity will show very little
Doppler shift, and will not be normally chosen.
If the new remote initially does not find a satellite with a very
positive frequency shift, then the new remote relaxes the Doppler
criterion and would accept a satellite with just a positive
frequency shift. If the new remote still does not find an
acceptable satellite, the new remote keeps relaxing the Doppler
condition until any satellite would suffice. In this instance the
new remote will chose a tentative channel with a near neutral or
negative Doppler.
This procedure selects a good tentative channel for the new remote
upon which the remote will receive a good signal. Most of the time
this procedure for selecting a positive Doppler shift also yields a
channel that will serve the remote for a maximum useful
duration.
Step IC
Will the new remote interfere with a previously assigned or old
satellite? Refer to FIG. 7 for a flow chart of the logic for Step
IC.
The new remote now monitors the channels near the tentative
channel. For example, if channel pair 43 is selected as a tentative
channel pair, the new remote will monitor channels 40 thru channels
46. The new remote will listen to the various old satellites, and
might hear as follows:
A. The new remote hears a signal on channel 42
B. The above signal originated on channel 44
C. The old satellite is listening on channel 46
D. The old satellite is receiving a signal at 32 DB
E. The new remote is receiving a signal at 28 DB
Consequently, the new remote can calculate that if it were to
transmit on channel pair 43 whether it would interfere with the
previously assigned receiver at the old satellite. In this
particular case, the old satellite is moving away from the new
remote (it hears a signal on 2 channels lower channel than was
transmitted). Consequently, if the new remote transmitted on
channel 43, it would be received at the old satellite on channel
41. However, since the old satellite is listening on channel 46
this does not pose a problem.
If the new remote would interfere with a previously assigned
satellite, the new remote would try another channel.
Step 1D
Will the new satellite be able to listen to the new remote on the
tentative channel? Refer to FIG. 8 for a flow chart of the logic
for step ID.
Next, the new remote attempts to signal the new satellite on the
proposed channel. The new remote already knows that it will not
interfere with any previous conversations.
If the new satellite hears the new remote, the new satellite knows
that all four tests have been completed, and that a conversation
between the new remote and the new satellite can begin. The new
satellite signals the new remote that all is well, and that
communications will soon commence.
If the new satellite can not satisfactorily hear the new remote,
the new satellite will not respond to the signal from the new
remote. If the new remote does not get a response, it assumes that
this channel is unsatisfactory, and the new remote attempts another
channel.
Part II
Establish satellite to ground station assignments.
Next, the new satellite must now find a ground station to accept
the call, and a channel also must be assigned in the satellite to
ground station band. In this part of the call set up the satellites
initiate the calls, and the land bases perform the initial tests to
determine tentative channels for signaling. The logic to set up
this link is similar to the logic to set up the mobile to satellite
link. Because the logic is essentially identical, it will not be
explained in the detail used for the mobile to satellite channel
set up logic.
Part IIA
Will a ground station interfere with a previously assigned
satellite receiver?
In this particular part of the call set up process, the various
ground stations are transmitting codes on those channels where the
ground stations know that they will not interfere with an existing
conversation. These transmissions also include the ground station
serial numbers. The ground stations observe the signals from the
various old satellites. The ground stations observe the various
signals and the frequency shifts and calculate in a manner as above
whether they can transmit signals on a given communication channel.
The method involves the various Doppler shift calculations in a
manner similar to the setting up of the other tentative
channel.
Part IIB
Will the new satellite hear a good signal from the new ground
station that will not be interfered with by an old ground
station?
Many times the various signal transmissions from the new ground
stations will be masked by other ongoing communications from old
ground stations. The satellite will only hear those signals from
the new ground station on channels that would be acceptable for
communications.
The satellite also keeps a current table of the ground station
serial numbers that it hears.
The satellite will only choose situations for the satellite to
ground link that have a positive Doppler shift in a manner as
described earlier.
Part IIC
Will the satellite interfere with a previously assigned or old
ground station?
The satellite searches for a tentative ground station channel. The
satellite initially only chooses a channel with a positive Doppler
shift to insure that the satellite is moving towards the ground
station.
The satellite now scans the entire range of channels within the
Doppler shift range; that is, the three channels above, and the
three channels below. If the satellite concludes that it would
interfere with an old ground station, the satellite searches for a
new tentative channel.
If the new satellite finds a channel upon which it would not
interfere with an old ground station, the new satellite signals the
new ground station on that tentative channel that it wants to begin
communications.
Part IID
Will the ground station hear a good signal from the new
satellite?
When the satellite signals the new ground station on the tentative
channel, the ground station will hear the transmission on a shifted
channel. If the ground station hears a good signal without actual
interference, the ground station knows that this is a good channel,
and completes the call set up process.
In U.S. Pat. No. 5,119,504 issued to Motorola, the satellite system
knows the location of the satellites and ground user. Consequently,
in U.S. Pat. No. 5,119,504 adjustments to transmitted frequencies
are precalculated to accommodate the Doppler shifts. In contrast to
that approach the approach described herein accepts the shifts,
does not keep track of either satellite locations or subscriber
locations, and lets the Doppler shifts occur. The approach
described herein accommodates the Doppler shifts as they occur, and
does not preplan for them.
Call Length Discussions
In today's cellular, conversations tend to be long and comfortable.
Although the cost per minute is high, the users of cellular
typically take their time to discuss a particular issue, and the
call is not limited to a short discussion. During the course of
normal cellular land based conversations, the mobiles frequently
move and become distant from their assigned base, and become close
to an alternate base. To provide better service, the mobiles are
usually "handed off" to the alternate base.
In the satellite system, where various satellites move at
tremendous speeds, the frequency reuse problem is continuously
changing. A satellite that is in a previous conversation can
suddenly appear over the horizon and interfere with an existing
conversation.
Since satellites typically move at speeds over 18,000 miles per
hour, we observe that in a single minute a satellite would move
over 300 miles. Consequently, the relative position of the
satellites above any given user will change dramatically during the
process of a normal call. Consequently, this system has a maximum
call segment length of one half minute before handoff must be
effected.
Various computer messages will naturally find that a half minute is
normally more than enough to meet their limited requirements. In
this system, voice conversation segments will be limited to one
half minute, and then they will be automatically forced to go
through the above call set up procedure for a handoff. Such
handoffs are common in cellular radio, and the user is typically
not even aware that he has been handed off to a different site. In
the satellite system, the user will not know that he has been
handed off to a different satellites.
Handoff Discussions
In the current operating land based cellular radio systems in the
United States, users are automatically forced into a handoff
situation when their signal quality degrades. Signal quality is
measured continuously, and when it falls below a certain
predetermined level, handoff procedures are initiated. Either a
weak signal caused by increasing distance from the concerned base,
or an interference situation can cause signal quality
degradation.
In the satellite system described herein, similar technology is
used, and when signal quality degrades, the handoff process is
initiated. Signal quality in the remote to satellites link could
degrade due to a satellite becoming to far away from either its
mobile user, or an interference situation. Signal quality in the
satellite to land base site link could degrade due to a satellite
becoming to far away from its land base, or an interference
situation.
There is a potential problem in that the call set up process could
be reinitiated, and a different land base might be involved. Since
the call is being routed into the land line network from a
previously chosen land site, the entrance of a new land base would
cause the call to be dropped. For example, a remote user located
near Chicago, Ill. could be routed through a Milwaukee, Wis. land
base site. However, upon reinitiating the call set up process, a
base site in St. Louis, Mo. could be chosen, and the call certainly
would be lost. Although the satellites are moving at tremendous
speeds, the relative location of the land based base site and the
land based mobile (remote) stay relatively constant. For example,
the user mentioned above who is near Chicago, would still be near
Chicago one minute later even though the concerned satellite would
have moved considerably.
Consequently, the system is designed such that various satellites
might be changed out, but the same base sites stay involved during
a handoff. During a handoff process, the remote will only consider
satellites that can reach the concerned base. This adds another
step in the logic in step Ib above. Refer to FIG. 9 for flowchart
of handoff logic.
In the statistically rare case that the mobile user can not find a
satellite that can reach his current base, the call is
automatically terminated. Because both the mobile to satellite link
and the satellite to land base link are chosen with good positive
Doppler shift, the operation of the system tends to pair mobile
users with land bases that are relatively close to each other.
Special Considerations
The approach described herein seeks to lower total system cost
though the elimination of several costly subsystems in conventional
LEOS systems. With a lower system cost, the ultimate cost for an
individual subscriber to make a telephone call, or receive a
digital message is lower. However, the design of this system
presents some potential problems in call set up that will translate
into high costs for the subscriber.
Consider a subscriber who lives in the mountains of Northern Italy,
who wants to use his subscriber phone to usually contact various
land line phones in Italy. Based upon the description of the
system, sometimes his calls will be received by the Italian base
station. In these particular instances, the cost to route his call
will not be severe. However, upon receipt of his phone bill, he
would note that some of his calls were routed through Greece,
others through Algeria, others even through Norway. In some
instances his calls would not even be completed because they might
be received by countries that have unsatisfactory phone
service.
In the handoff scenario, it was observed that the process of
continuation of a call through handoff dictated that a specific
base site be included in the new links. The solution to the
complexities of system use for the subscriber in Northern Italy is
handled in a similar manner. Each subscriber unit can be manually
programmed to accept only certain base sites. The paper directions
that are supplied with the handset show the locations of the base
sites, and these directions explain the call routing problems. Each
subscriber has the option of letting the satellite system select
the base site, or letting the subscriber unit limit the choice to
base sites to locations that will limit the call interconnect
charges. Entering the numbers of the acceptable base sites into the
subscriber unit is a small inconvenience compared to the phone
charges that would otherwise ensue.
This process of entering the numbers of the acceptable base sites
would also be important in the United States. A user located in
central Illinois who wanted to call Chicago would naturally prefer
that the call be routed through the Chicago base site. If the call
were routed through the base site in Kansas city, the costs would
be higher. This is an optional feature in the subscriber unit that
will help contribute to lower costs for the system user.
The method of assigning channels as described herein can be used
with any pattern of directional or beam antennas. Directional
antennas permit an enhanced amount of frequency reuse. Directional
receiving antennas on the satellites will provide grin, and
consequently, a weaker signal will be acceptable. Directional
transmitting antennas on the satellites will permit a greater
signal power to be directed locally to a limited area on earth
which will provide the earth subscriber units with a better
signal.
The preferred method of utilizing directional antennas would be to
divide the area below a satellite into four different quadrants.
One fourth of the channels on board the satellite would be assigned
to each quadrant. An alternative embodiment of the inventive system
will now be described. This new embodiment removes the orbit
control rockets, orbit control system, and large amounts of rocket
fuel from the satellites, and lets the satellites orbit without
earth control, or actually orbit in a free or non controlled
manner. Naturally, a satellite in orbit traverses a carefully
defined orbit based upon its initial position and velocity.
However, by removing earth control, and intentionally creating
diverse orbits for the individual satellites, the satellite orbits
with respect to each other will appear to be random. To a user on
the surface of the earth, the various satellites above going in
different directions will appear to be random. FIG. 3 shows a
typical group of satellites that might be overhead of a specific
location on earth. Consequently, we shall refer herein to such as a
system as a random orbit satellite system.
The small rockets and onboard control system that keep the
satellite antennas pointed towards earth remain in the system.
DETAILED DESCRIPTION OF THE INVENTION
Presently, most of the proposed LEOS satellites systems are grid or
pattern style systems wherein the satellites orbit in a preplanned
pattern. We have found that our present system for channel
assignment operates equally well for a pattern style system as it
does for a system where the satellites are in a random orbit as
defined above.
The strategy of a random orbit satellite telephone system consists
of orbiting 200 smaller satellites instead of the proposed 70
satellites. Since these satellites will orbit in a random manner, a
ground or earth controller need not be present to control the
orbits of the satellites.
Since these satellites will not need orbit correction abilities,
the various rockets, computer control system, and large amounts of
rocket fuels will not be present in the satellites. Many of the
complicated features present in U.S. Pat. No. 5,303,286 will be
simply eliminated. This will achieve a significant reduction in the
weight, size, complexity, and cost of the individual satellites
while providing better results.
Because of the random nature of the system, redundancy is built in.
The failure of any one unit does not significantly alter the system
operation. Back up satellites are not required.
In addition, since there will be more satellites, the amount of
radio telephone equipment on each satellite will be reduced.
Additional radios for extra capacity can be added to the system by
merely launching additional satellites to increase the number
beyond 200 units.
The weight per satellite is reduced from 1,500 pounds to less than
200 pounds per unit because of the elimination of rocket mechanisms
and orbit control mechanisms. These 200 units at 200 pounds each
make a total weight of 40,000 pounds which is about one third of
the weight of a standard proposed LEOS system. This reduction in
launch weight will considerably reduce the launch complexity and
cost. In addition, a further reduction in weight will be obtained
because broad band amplifiers are now being perfected and will be
available in the near future. These amplifiers will permit the
elimination of a radio for each channel. Instead of a radio for
each channel, a single broad band amplifier can amplify many
signals simultaneously. This will permit another significant weight
and cost reduction.
Furthermore, because the inventive system does not need an orbit
control system, the engineering work to design the inventive system
will be also reduced considerably. A basic advantage is that a
master ground or earth station does not have to be designed; and
further an orbit control system onboard each satellite does not
have to be designed.
The following are the operating system parameters:
______________________________________ Number of Satellites in
Random Orbit 200 Units Typical Height of a Satellite 500 Miles
Number of Base Sites 1 Minimum Operating Frequency of System 1,800
Megahertz Spectrum set aside for total system: 42 Megahertz
Spectrum for up link for remote unit 10 Megahertz Spect. for down
link for satellite to remote 10 Megahertz Spect. for up link for
landline network 10 Megahertz Spect. for down link by satel. to
land line 10 Megahertz Spectrum for paging satellite to satellite 1
Megahertz Spectrum for paging satellite to ground 1 Megahertz
Channel spacing 15 Kilocycles Guard Bands at end of each block 50
Kilocycles Number duplex channel pairs in the system 650 for
satellite to remote: (650 channels talk up, 650 channels down)
Number of duplex channel pairs in the system 650 for satellite to
land line (650 channels talk up, 650 channels down) Duplex channel
pairs required to per call 2 Number of Satellites in initial launch
200 Number duplex pairs per satellite (mobiles) 40 Number duplex
pairs per satellite (landline) 40 Times a channel typically is
reused 5 Paging channels (single) 40
______________________________________
The present system can support about 3,250 different simultaneous
phone calls. The 650 channel pairs can be used once in a given
geographic area. A reuse factor of five yields 3,250.
Time division multiplexing is incorporated in this system to
enhance system capacity. Voice conversations can be compressed into
small time slots, and several conversations can be put on a single
channel. For purposes of explaining the paging and signaling and
call set up process, a single channel assignment is described.
Moreover, the process of time division multiplexing is incorporated
in the system by considering each time slot on a channel, as a
logical channel in the following explanation. System Assures
Excellent Communication Coverage.
A satellite in an orbit 500 miles high sees to a horizon about 1937
miles away. For example, a single satellite over either Missouri or
over Nebraska will cover most of the United States. In fact, a
satellite at an altitude of 500 miles will cover a square area 12
million square miles. The surface of the planet earth is about 197
million square miles. Consequently, a single satellite can cover
about six percent (6%) of the surface area of the planet.
The distance to the horizon is a concept that was developed by the
ancient sailors. As one sails away from a lighthouse, it does not
fade into the distance, but actually drops below the horizon. These
calculations are based on straightforward trigonometry. This
distance to horizon concept was key in the development of the
initial cellular radio system. Frequency reuse was guaranteed in
that potential offending transmissions are below the horizon. We
present herewith a table for distance to the horizon for various
satellite altitudes. The following calculations are based on a
perfect sphere, and consequently, are only approximate.
______________________________________ Altitude Distance to Horizon
______________________________________ 100 Miles 968 Miles 500
Miles 1,937 Miles ______________________________________
It is observed from the above table that increasing the altitude of
the satellites from 100 to 500 miles by a factor of five only
doubles the distance to the horizon. When a satellite has a
distance to the horizon of 1,937 miles, it covers an area of
approximately Pi times Radius squared.
Since the surface area of planet earth is 197 million square miles,
we can see that a single satellite 500 miles high covers about 6
percent of the earth's surface.
The 200 satellites in random orbits will on average cover the earth
12 times over (200 times 6 percent)! In fact, on average a citizen
in Chicago will be able to observe on average about 12 satellites
in the sky above him. If he was lucky, he might see as many as 14
or even 15, and if he was unlucky, he would see perhaps only 9 or
10 satellites.
The large number of satellites available to an earth user
guarantees that the user will be able to select a satellite that
will provide a strong signal. In the grid or pattern systems with
fewer satellites, a user is frequently assigned to a satellite that
is not directly overhead. Consequently, even with less weight in
orbit, the user of the random orbit satellite system is guaranteed
a better signal than would be received from a grid system.
Since an individual satellite covers 6% of the earth's surface, the
chance that an individual satellite will not cover a specific spot
on earth is 94%. With 200 satellites in random orbits, the chance
that a particular spot on earth will not be covered by any
satellite can be calculated as 0.94 to the 200th power.
Consequently, the chance that a single place on earth will not be
covered by any of 200 satellites is about 0.0004 percent. For our
Chicago user, this means that the chance that he would not see a
satellite is only 4 chances in a million.
If, however, there were only 180 units in operation, the chances
that a particular spot would not be covered is 0.94 to the 180
power. This calculation yields 0.0000145. Consequently, the chance
that a given spot on earth would not be covered 14 in a
million.
If however, there would be only 150 units in operation, the chances
that a particular spot on earth, Chicago for example, would not be
covered would be 0.94 to the to the 150 power. This calculation
yields 93 chances in a million that a spot would not be covered.
The following table shows the relative effectiveness of different
numbers of units in a random orbit satellite system.
______________________________________ Number of Units Chances of
not covering Chicago ______________________________________ 200 4
in a million 180 14 in a million 150 93 in a million 120 596 in a
million 100 2054 in a million
______________________________________
The system will perform satisfactorily with any number of units
over 180. If significantly less than 180 units are used, the
chances of a point on earth not having coverage at any time becomes
too large. If over 230 units are used, the system performs better,
but the cost advantage of using random satellites no longer is as
significant. Consequently, 200 units are chosen as the initial
number of units to be launched. As various units fail, the system
still performs very satisfactorily until at least 10% of the units
have failed. 180 units still provides a very good chance of
covering a particular spot.
A satellite in an orbit 500 miles high will be required to provide
coverage out to an angle of about 62 degrees to reach the horizon,
see FIG. 10. Even at that long distance, radio propagation is
adequate for suitable communications. However, it is known that
earth objects such as buildings and mountains would frequently
interfere with the propagation path.
Moreover, because of the fact that many of the visible satellites
for a Chicago earth user typically might be near the horizon and
not provide very good radio coverage because of the building
shadows, it should be understood that usually only 1 or 2
satellites will be in position to provide very good radio coverage.
Consequently, the chances of not having a satellite in a good
position are somewhat greater than the chances of not being able to
see any satellites.
The various buildings in Chicago are a severe propagation handicap.
Users in Chicago need as many visible satellites as possible.
Consequently, the random orbit satellite system is more apt to have
a unit directly overhead than the grid system. Because the
inventive system has approximately three times the satellites as
compared to known technology, the probabilities of one or more
satellites being in a position to provide coverage around building
shadows, as occur in cities such as Chicago, is greatly
enhanced.
Because of potential coverage problems it is important to make very
careful radio frequency channel assignments. As is explained
herein, the Doppler shift is used to insure that only assignments
will be made where the radio propagation will improve during the
conversation. Only when a positive Doppler is not available as
explained herein, will the system use a negative Doppler
alternative. This key feature of the invention helps compensate for
the chances of not having a satellite directly overhead.
Satellite Launch
One satisfactory, yet expensive method of launching 200 satellites
into 200 different random orbits would be to launch each individual
satellite into a different orbit. These orbits can be created in
advance by off line computers that would generate 200 different
orbits. Each of the satellites would be launched individually into
the preplanned orbit. However, technology as explained in U.S. Pat.
No. 5,295,642 which discusses airborne launching systems could be
used to control the cost.
However, if a single launch vehicle were used, and the satellites
were jettisoned sequentially from a single space platform, they
would end up orbiting the earth in a cluster. A potential user in
Chicago would typically see zero satellites overhead, but very
rarely he might see 200 of them at once. Such a system is clearly
unsatisfactory.
However, if a single launch vehicle were used, and the satellites
were jettisoned at slightly different velocities, their orbits
would still be great circles, and would only be slightly inclined
from the orbit of the launch vehicle. Such a system is clearly
unsatisfactory.
However, there are various alternatives to create patterns of
satellites in fixed orbits that would appear like satellites in
random orbits to earth users. Thus although the satellites would be
in well defined orbits with a well defined plan, they would appear
to be random to an earth based user.
A lower cost approach other than launching 200 satellites into 200
different random orbits method is to create 20 different planes
similar to those explained in U.S. Pat. Nos. 5,274,840 and
5,161,248. In each of the planes there would be 10 different
satellites. The satellites in each plane would move at different
velocities, and consequently each of the satellites in each plane
would be position independent from the other satellites.
Consequently, the relative position of each satellite in its plane
would appear to an earth user to be random. Because there would be
so many different planes, the satellites would appear to be random
to the earth user.
By choosing only 20 different planes as opposed to 200 different
random orbits, the system launch complexity is greatly reduced. The
10 satellites in each plane can be launched as a group. To create
slightly different orbits with different velocities and altitudes
in a single plane can be achieved easily with well known
technology.
In the grid approach it is imperative to keep the satellites at
precisely the same altitude so that they orbit at the same speed
and maintain the grid pattern. However, in the inventive approach
altitude is not important. Individual satellites can be at various
different altitudes, and this does not impair satisfactory system
performance.
Consequently, in the initial launch the satellites in each plane
are placed at slightly different altitudes with different
velocities. Although the chances of collision are low, the
intentional placement of the satellites at different altitudes
forces the chances to be nil. Consequently, the individual
satellites in a given plane will appear to be randomly located with
respect to each other. The following table shows the altitude and
velocity of four of the 10 satellites in a given orbit plane.
______________________________________ Satellite number Altitude
Approximate Velocity ______________________________________ 1A 500
miles 18,769 mph 2A 502 miles 18,773 mph 3A 504 miles 18,777 mph 4A
506 miles 18,781 mph to 10A etc. etc.
______________________________________
It is observed that the velocity difference of the satellites in
different orbits only 2 miles apart is about 4 miles an hour. The
method to create these different orbits would be to launch a single
satellite from a space craft, and then the space craft would fire
on board rockets to raise its altitude by two miles and its
velocity by four miles an hour. Then another satellite would be
released.
Initially, these satellites would orbit in a cluster, but as time
increases, they would naturally separate. Two satellites in orbits
2 miles apart would separate at about 4 miles an hour. It would
take one day to separate them about 100 miles. A month would
separate them by about 3,000 miles. Four months would separate them
by 12,000 miles. Naturally, these orbits should not be exactly 2
miles apart, or some pattern might develop that would be
detrimental to system coverage.
The non adjacent satellites in any given plane would naturally
separate at an even faster rate. After several months in orbit, all
the satellites within any given plane would essentially appear to
be in random positions within the given plane.
One of the advantages of this non patterned approach is that
additional satellites can be placed into orbit at a later time, and
they can be placed in almost any low earth orbit at any altitude,
and they would not hurt, but strengthen system operation. For
example, 10 new satellites could be piggybacked on an entirely
different launch program, and just be added to the system.
Channel Assignment
The channel assignment procedure in this embodiment is as in the
previous embodiment.
While the invention has been particularly shown and described with
reference to preferred embodiments thereof, it will be understood
by those skilled in the art that various changes in form and
details may be made therein without departing from the spirit and
scope of the invention.
* * * * *